Buggy code:
public class MailReaderSamplerGui extends AbstractSamplerGui implements ActionListener , FocusListener { private static final long serialVersionUID = 240L ; private JTextField serverTypeBox ; private JTextField serverBox ; private JTextField portBox ; private JTextField usernameBox ; private JTextField passwordBox ; private JTextField folderBox ; private JLabel folderLabel ; private JRadioButton allMessagesButton ; private JRadioButton someMessagesButton ; private JTextField someMessagesField ; private JCheckBox deleteBox ; private JCheckBox storeMimeMessageBox ; private final String ServerTypeLabel = JMeterUtils . getResString ( "mail_reader_server_type" ) ; private final String ServerLabel = JMeterUtils . getResString ( "mail_reader_server" ) ; private final String PortLabel = JMeterUtils . getResString ( "mail_reader_port" ) ; private final String AccountLabel = JMeterUtils . getResString ( "mail_reader_account" ) ; private final String PasswordLabel = JMeterUtils . getResString ( "mail_reader_password" ) ; private final String NumMessagesLabel = JMeterUtils . getResString ( "mail_reader_num_messages" ) ; private final String AllMessagesLabel = JMeterUtils . getResString ( "mail_reader_all_messages" ) ; private final String DeleteLabel = JMeterUtils . getResString ( "mail_reader_delete" ) ; private final String FolderLabel = JMeterUtils . getResString ( "mail_reader_folder" ) ; private final String STOREMIME = JMeterUtils . getResString ( "mail_reader_storemime" ) ; private static final String INBOX = "INBOX" ; public MailReaderSamplerGui ( ) { } public String getLabelResource ( ) { } @ Override public void configure ( TestElement element ) { } public TestElement createTestElement ( ) { } public void modifyTestElement ( TestElement te ) { } private void init ( ) { setLayout ( new VerticalLayout ( 5 , VerticalLayout . BOTH , VerticalLayout . TOP ) ) ; setBorder ( makeBorder ( ) ) ; add ( makeTitlePanel ( ) ) ; JPanel serverTypePanel = new JPanel ( ) ; serverTypePanel . add ( new JLabel ( ServerTypeLabel ) ) ; serverTypeBox = new JTextField ( 20 ) ; serverTypeBox . addActionListener ( this ) ; serverTypeBox . addFocusListener ( this ) ; serverTypePanel . add ( serverTypeBox ) ; add ( serverTypePanel ) ; JPanel serverPanel = new JPanel ( ) ; serverPanel . add ( new JLabel ( ServerLabel ) ) ; serverBox = new JTextField ( 20 ) ; serverPanel . add ( serverBox ) ; add ( serverPanel ) ; JPanel portPanel = new JPanel ( ) ; portPanel . add ( new JLabel ( PortLabel ) ) ; portBox = new JTextField ( 20 ) ; portPanel . add ( portBox ) ; add ( portPanel ) ; JPanel accountNamePanel = new JPanel ( ) ; accountNamePanel . add ( new JLabel ( AccountLabel ) ) ; usernameBox = new JTextField ( 20 ) ; accountNamePanel . add ( usernameBox ) ; add ( accountNamePanel ) ; JPanel accountPassPanel = new JPanel ( ) ; accountPassPanel . add ( new JLabel ( PasswordLabel ) ) ; passwordBox = new JTextField ( 20 ) ; accountPassPanel . add ( passwordBox ) ; add ( accountPassPanel ) ; JPanel folderPanel = new JPanel ( ) ; folderLabel = new JLabel ( FolderLabel ) ; <START_BUG> folderBox = new JTextField ( MailReaderSamplerGui . INBOX , 10 ) ; <END_BUG> folderPanel . add ( folderLabel ) ; folderPanel . add ( folderBox ) ; add ( folderPanel ) ; HorizontalPanel numMessagesPanel = new HorizontalPanel ( ) ; numMessagesPanel . add ( new JLabel ( NumMessagesLabel ) ) ; ButtonGroup nmbg = new ButtonGroup ( ) ; allMessagesButton = new JRadioButton ( AllMessagesLabel ) ; allMessagesButton . addChangeListener ( new ChangeListener ( ) { public void stateChanged ( ChangeEvent e ) { if ( allMessagesButton . isSelected ( ) ) { someMessagesField . setEnabled ( false ) ; } } } ) ; someMessagesButton = new JRadioButton ( ) ; someMessagesButton . addChangeListener ( new ChangeListener ( ) { public void stateChanged ( ChangeEvent e ) { if ( someMessagesButton . isSelected ( ) ) { someMessagesField . setEnabled ( true ) ; } } } ) ; nmbg . add ( allMessagesButton ) ; nmbg . add ( someMessagesButton ) ; someMessagesField = new JTextField ( 5 ) ; allMessagesButton . setSelected ( true ) ; numMessagesPanel . add ( allMessagesButton ) ; numMessagesPanel . add ( someMessagesButton ) ; numMessagesPanel . add ( someMessagesField ) ; add ( numMessagesPanel ) ; deleteBox = new JCheckBox ( DeleteLabel ) ; add ( deleteBox ) ; storeMimeMessageBox = new JCheckBox ( STOREMIME ) ; add ( storeMimeMessageBox ) ; } @ Override public void clearGui ( ) { } private void initGui ( ) { } public void actionPerformed ( ActionEvent e ) { } public void focusGained ( FocusEvent e ) { } public void focusLost ( FocusEvent e ) { } }

Fix:
folderBox = new JTextField ( MailReaderSamplerGui . INBOX , 20 ) ;